Fuses for elevated circuit voltages require often fusible elements whose length exceeds that of the casing or fuse tube. There are two widely used ways of complying with this requirement. One of them consists in imparting an undulating shape, and the other consists in imparting a helical shape, to the fusible element. When deciding on the helical configuration for the fusible element, the latter is generally wound around a heat resistant insulating support or mandrel. Fusible elements which have an undulating shape are generally not provided with insulating supports. Unsupported fusible elements whose shape is undulating are generally only applied for relatively short fuses, or fuses intended to have a relatively limited voltage rating, e.g. a few kilovolts. The absence of any support for undulating fusible elements gives rise to the danger that the elements will be distorted by the impact of the granular arc-quenching filler when the latter is filled into the casing. To avoid or minimize this danger it is common practice to increase as much as possible the dimensional stability of unsupported undulating fusible elements by imparting a relatively considerable thickness to them. This, however, results in a relatively small area of interface between the surface of the fusible element and that of its surrounding granular arc-quenching medium. In order to optimize the cooling action of the granular arc-quenching medium upon the metal vapors resulting from vaporization of the fusible element, the aforementioned interface should be as large as possible, and the thickness of the fusible element ought, therefore, to be as small as possible.
It is the prime object of this invention to provide electric fuses having undulated fusible elements which are not subject to the aforementioned limitations.
Other objects of the invention will become apparent as this specification proceeds.